In U.S. Pat. No. 3,949,020, issued Apr. 6, 1976, there is disclosed and claimed a process for the preparation of thermoplastic block copolymers by contacting conjugated diolefinic monomers mixed with divinylbenzene, under solution polymerization conditions, with a polystyryllithium catalyst, the amount of said divinylbenzene varying from about 0.5/1 to about 25/1 of divinylbenzene/active lithium molar ratio, a more preferred range is 2/1 to 10/1, whereby the resulting thermoplastic block copolymer is a non-gel and has the divinylbenzene coupling agent attached to the diolefinic portion of said copolymer block.
The procedure disclosed in U.S. Pat. No. 3,949,020 results in thermoplastic elastomeric block copolymers. These block copolymers are usually known as ABA or as SBS or SIS (polystyrene-polybutadiene-polystyrene or polystyrene-polyisoprene-polystyrene) block copolymers. The molecular weight of the polystyrene portions may range from about 7000 to about 50,000, the diolefinic portion can range from about 10,000 to about 100,000. Because the block polymers prepared in accordance with the process are branched, they process much more readily than their linear counterparts. They are useful in the preparation of a host of finished products ranging from relatively soft, weak tensile adhesive substrates through thermoplastic elastomer products and even into plastic materials. These block copolymers can be heated, molded or shaped and then allowed to cool to a tough, useable material which exhibits properties of cured or vulcanized polymers, even though they are not required to be vulcanized. Shoe soles are an example. Other uses for such polymers are in adhesives and in films.
The use of an organolithium as an initiator results in a polymer chain that is described as "living". By the term "living polymer" is meant the product of a polymerization which has no termination or transfer reaction. Thus, the polymer chains have lithium atoms attached to the chain and even when all the monomer is consumed, if additional monomer is added, the polymerization will continue with the new monomer adding on to the already existing chains until it also is consumed. Thus, in the simplest case possible, in using the usual coupling agents, two living polymers, 2(A-B-), can be coupled to give a polymer having a molecular weight equal to the sum of the polymers (A-B-B-A). With a living polymer system, it is necessary to have a system free of impurities in order to avoid termination of the growing polymer chain with adventitious impurities found in the usual coupling agents. There is much disclosure in the prior art on coupling living polymers using coupling agents. Also, there are many problems associated with coupling polymers in a living polymer system. When coupling a lithiated system, the most serious concern of those skilled in the art is that a diblock polymer (A-B) will result rather than many block segments being connected by the coupling agent, if the lithium should prefer to react with some other material, i.e., an impurity rather than with the coupling agent.
U.S. Pat. No. 3,639,517, issued Feb. 1, 1972, discloses a process for the preparation of resinous branched copolymers comprising sequential steps of (A) contacting under polymerization conditions a monovinyl substituted aromatic hydrocarbon such as, for instance, styrene in an amount to provide from 40 to 90 weight percent of the total monovinyl substituted aromatic hydrocarbon employed in preparing the copolymer for a time sufficient to polymerize substantially all of the monovinyl substituted aromatic hydrocarbon; then (B) charging to the polymerization reaction product of step (A) the remaining monovinyl substituted aromatic hydrocarbon monomer representing from 10 to 60 weight percent of total monovinyl substituted aromatic hydrocarbon monomer employed in preparing said copolymer, adding additional organolithium initiator, and (C) charging to the polymerization reaction product of step (B) a conjugated diene monomer such as, for instance, butadiene, and polymerizing to essential completion the diene monomer in the presence of said reaction product of step (B) to form a block copolymer and (D) charging to the polymerization reaction product of step (C) a polyfunctional treating agent capable of reacting the terminal lithium atoms on a preformed polymer to form a branched polymer wherein said polyfunctional treating agent is employed in an amount to provide about 0.05 to 2 equivalent of the polyfunctional treating agent per gram atoms of lithium. It is said that the polyfunctional treating agent can be polyisocyanate such as benzene-1,2,4-triisocyanate, polyepoxide, such as epoxidized linseed oil, a polyketone or a polyvinyl aromatic compound such as divinylbenzene.
The invention of U.S. Pat. No. 3,949,020 overcomes the problems of impurities entering into the system by what was then an unobvious and novel method of block polymer preparation. The polymerization system of U.S. Pat. No. 3,949,020 does not expose the polymerization system to the chance deactivation of the polymer-lithium moiety by the impurities. The coupling agent is introduced into the polymerization system at the beginning of the diolefin polymerization rather than after the living polydiolefin-lithium chains have been formed and, therefore, susceptible to premature termination.
There is disclosed in U.S. Pat. No. 3,363,659, issued Jan. 16, 1968, the polymerization of butadiene with an alkyl lithium catalyst and the use of from about 0.025 to about 0.4 parts by weight of a comonomer such as divinylbenzene.
In U.S. Pat. No. 3,855,189, issued Dec. 17, 1974, there is disclosed the preparation of a random copolymer of, for instance, butadiene and styrene in which (1) introducing into a polymerization zone at least on polymerizable monomer selected from the group of conjugated dienes, polymerizable monovinyl-substituted aromatic compounds and mixtures, under polymerization conditions employing organolithium initiator; (2) polymerizing said polymerizable monomer with said organolithium initiator, forming polymer-lithium moieties; (3) treating the resulting polymerization mixture with at least one polar compound in a minor amount effective to improve coupling of said polymer-lithium, wherein said polar compound is ether, thioether, tertiary amine or triazine, or mixtures thereof; (4) treating said polymerization reaction system with at least one polyvinyl aromatic compound in a minor amount effective to couple said polymer-lithium moieties, wherein said polar compound is added to the polymerization mixture prior to or coincidentally with said polyvinyl aromatic compound. This patent teaches the use of commercial divinylbenzene as the polyvinyl aromatic compound.
The present invention distinguishes from the process of the aforementioned patent in that the disclosure in the aforementioned patent is the formation of random copolymers a diolefin monomers and vinylaromatic compounds or, as is indicated, the preferred polymers include polybutadiene, rubbery butadiene-styrene copolymers of low vinyl unsaturated content. The aforementioned patent suggests that many things, such as potassium salts of alcohols or phenols can be employed to randomize the copolymerization of conjugated dienes and monovinyl substituted aromatic compounds. Furthermore, the process of the present invention distinguishes from this reference in that the polymerization of this reference is complete prior to the addition of the polyvinyl aromatic compound. If such order of addition were employed in the process of the present invention, there would be a serious problem of deactivating the polystyrene-polybutadiene-lithium block prior to coupling of two or more SB or AB blocks to form SBS or ABA block polymers.
However, the process of U.S. Pat. No. 3,949,020 is not without its deficiences.
Due to the fact that some residual unreacted divinylbenzene and also due to the fact that commercial divinylbenzene, a mixture of isomers, is a relatively impure product, usually containing up to not more than 55 percent divinylbenzene, with ethylvinylbenzenes (EVB) and diethylbenzenes (DEB) being the major impurities, there is a residue of divinylbenzene, ethylvinylbenzene and diethylbenzene left in the thermoplastic block polymers after they have been isolated, which causes a slight odor to remain in the block polymers.
A typical example of the preparation of a block copolymer by the process of U.S. Pat. No. 3,949,020 is that a polystyryllthium initiator of 12,500 molecular weight was prepared by polymerizing styrene with butyllithium. For instance, 200 milliliters (ml) of styrene and 700 ml of cyclohexane were passed through a silica gel bed and sparged with nitrogen. Afterwards, there was contained 33.3 grams (g) of styrene in each 170 ml of solution. To this solution was added 2.6 ml of 1.05 N-butyllithium and the resulting polymerization resulted in a polystyryllithium of 12,500 molecular weight and a 0.0152 normality. Liquified butadiene (350 ml) and 2650 ml of cyclohexane were passed through a silica gel bed and sparged with nitrogen gas, resulting in a solution having 8.4 g of butadiene per 165 ml of solution. Into a series of bottles was placed 165 ml of solution of cyclohexane containing 8.4 g of butadiene. To each of these bottles was added 0.4 ml of 0.2 N-butyllithium to act as a scavenger, after having added 0.3 ml of divinylbenzene (3.78 molar of a 55 percent solution of divinylbenzene dried over calcium sulfate, giving a DVB/lithium ratio equal to 5/1). Subsequently, there was added 15 ml of the above prepared polystyryllithium giving a polybutadiene having a kinetic molecular weight of 40,000. The solution was allowed to react for 40 minutes at 65.degree. C., after which a methanol solution of a phenolic antioxidant was added to the mixture to stop the further polymerization. The polymer was isolated, air dried, and then vacuum dried to give approximately 99.6 percent by weight yield. The polymer was clear and colorless. It had a dilute solution viscosity in toluene at 30.degree. C. of 1.1 and the percent gel was 3.8. The polymer was readily soluble in benzene. When molded or remolded at 149.degree. C., the polymer had a tensile strength of 19581.8 kPa at 905 percent elongation. However, such a polymer does present a small problem in that there is a slight odor, reminiscent of divinylbenzene.
It has been determined that up to about one-third of the divinylbenzene (DVB) remains unreacted after a practical polymerization time has elapsed. This remaining unreacted divinylbenzene results in an odor problem during the finishing operation of isolating the polymer from solution and a slight residual odor associated with the dried polymer.
Thus, it is desirable to, in some manner, increase the efficiency of the divinylbenzene coupling reaction. It is also desirable that the level of the divinylbenzene required in the coupling reaction be lowered. If such could be accomplished, there would be a resulting lowering of the material cost and a reduction in the residual odor of the polymer.